Embryonic development of multicellular organisms is a highly ordered process that requires coordination of individual cells. Every cell must decipher the numerous signals it receives and then properly execute commands in order to achieve the correct position and differentiated state in the animal. The exquisite controls over cell growth, determination, migration and adhesion are mediated by molecules located on the plasma membrane surface.
A class of membrane associated molecules known to regulate cellular interactions are receptor tyrosine kinase proteins. The evolutionary conservation of genes encoding receptor tyrosine kinase proteins and their targets has emphasized the importance of these proteins in intracellular communication, and has also provided model systems for genetic analysis of tyrosine kinase signalling pathways. Such studies have shown that some tyrosine kinases function to specify a particular cell fate, such as the sevenless (sev) receptor in Drosophila R7 photoreceptor cells and the Let-23 receptor in nematode vulval cells (reviewed by Greenwald and Rubin, Cell 68:271-281, 1992). The binding of sev with its ligand, boss, results in cell clustering suggesting a role in cell--cell adhesion for these molecules (Kramer et al., Nature 352:207-212, 1991). The receptor tyrosine kinase encoded by torso functions in pattern formation by specifying the terminal poles of Drosophila embryos (Sprenger et al., Nature 338:478-483, 1989). Genetic analysis has recently provided insight into the functions of a small number of receptor tyrosine kinases in mouse development, including the .alpha.-platelet-derived growth factor receptor, the colony stimulating factor-1 receptor, and c-Kit/W (Pawson and Bernstein, Trends in Genetics 6:350-356, 1990).
A growing number of closely related transmembrane receptor tyrosine kinase proteins containing cell adhesion-like domains on their extracelluar surface have recently been identified. Collectively, this group of proteins defines the Eph/Elk/Eck subfamily, which is made up of at least fifteen related but unique gene sequences in higher vertebrates (Hirai et al., Science 238:1717-1720, 1987; Letwin et al., Oncogene 3:621-627, 1988; Lindberg et al., Mol. Cell. Biol. 10:6316-6324, 1990; Lhotak et al., Mol. Cell. Biol. 11:2496-2502, 1991; Chan and Watt, Oncogene 6:1057-1061, 1991; Lai and Lemke, Neuron 6:691-704, 1991; Pasquale, Cell Regulation 2:523-534, 1991; Sajjadi et al., New Biologist 3:769-778, 1991; Wicks et al., PNAS 89:1611-1615, 1992; Gilardi-Hebenstreit et al., Oncogene 7:2499-2506, 1992; Bohme et al., Oncogene 8:2857-2862, 1993; Sajjadi and Pasquale, Oncogene 8:1807-1813, 1993). Eph family members encode a structurally related cysteine rich extracelluar domain containing a single immunoglobulin (Ig)-like loop near the N-terminus and two fibronectin III (FN III) repeats adjacent to the plasma membrane. Examples of Eph family members include Cek5 (Pasquale, Cell Regulation 2:523-534, 1991) and Erk; (Chan and Watt, Oncogene 6:1057-1061 1991). Another Eph family member, Sek, has been shown to be segmentally expressed in specific rhombomeres of the mouse hindbrain (Nieto et al., Development 116:1137-1150, 1992). The presence of cell adhesion-like domains in this family of tyrosine kinases suggests that these proteins function in cell-cell interactions.
The other major families of proteins implicated in cell adhesion include the cadherins, selecting, integrins, and those of the immunoglobulin superfamily (reviewed by Hynes, R. O. and Landers, A. D., Cell 68, 303-322, 1992). The extracelluar regions of cell adhesion molecules frequently contain peptide repeats, such as FN III motifs, epidermal growth factor (EGF) repeats, or Ig loops that may direct protein-protein interactions at the cell surface. A number of cell adhesion molecules in both vertebrates (Dodd, J. and Jessell, T. M., Science, 242, 692-699, 1988; Jessell, T. M., Neuron, 1, 3-13, 1988; Furley et al., Cell 61, 157-170, 1990; Burns et al., Neuron, 7, 209-220, 1991) and invertebrates (Bastiani et al., Cell 48:745-755, 1987; Elkins et al., Cell 60:565-575, 1990; Grenningloh et al., Cold Spring Harb, Symp. Quant. Biol. 55, 327-340, 1991; Nose et al., Cell 70:553-567, 1992) have been implicated in axonal growth cone guidance and pathway/target recognition. Other aspects of neuronal morphogenesis involving cell-cell interactions may also require the activities of cell adhesion molecules (Edelman and Thiery, In The Cell in Contact: Adhesions and Junctions as Morphogenetic Determinants, Wiley, New York, 1985; Hatta et al., Dev. Biol. 120:215-227, 1987; Takeichi, Development 102:639-655, 1988; Takeichi, Annu. Rev. Biochem. 59:237-252 1990; Takeichi, Science 251:1451-1455, 1991; Edelman, Biochemistry 27:3533-3543, 1988; Grumet, Curr. Opin. Neurobiol. 1:370-376, 1991; Hynes and Lander, Cell 68:303-322, 1992). For example, ectopic N-cadherin expression during gastrulation stage Xenopus embryos has been shown to interfere with segregation of the neural tube from the ectoderm (Detrick et al., Neuron 4:493-506, 1990; Fujimori et al., Development 110:97-104, 1990). Although many different types of cell adhesion molecules have been identified, little is known about how these adhesive interactions are regulated and how they function in cell signalling pathways during normal development.
A critical stage in the development of the nervous system is the projection of axons to their targets. Navigational decisions are made at the growth cones of the migrating axons. As axons grow their growth cones extend and retract filopodia and lamellipodia processes which are implicated in the navigational decisions and pathfinding abilities of migrating axons. Like peripheral nervous system axons, the growth cones of neurons associated with the central nervous system follow stereotyped pathways and apparently can selectively chose from a number of possible routes (reviewed by Goodman and Shatz, Cell 72:77-98, 1993). Early pathways in the vertebrate embryonic brain are thought to be arranged as a set of longitudinal tracts connected by commissures. However, the molecular mechanisms that underly growth cone navigation and axon pathfinding in development are poorly understood (Hynes, R. O. and Lander, A. D., 1992, Cell 68:303).
Evidence indicates that the development of the endolymphatic duct is under the control of neuronal induction (Van De Water and Represa, Van De Water, T. R. and Represa, J. (1991). Ann. NY Acad. 630:116-128, 1991). The endolymphatic duct pinches off from the otic vesicle and elongates to form a tube that apparently functions in regulating the endolymph fluid pressure in the membranous labyrinth of the internal ear (Guild, Amer. J. Anat. 39:57-81, 1927; Rugh, The Mouse: Its reproduction and Development. Minneapolis: Burgess, 1968; Sher, 1971; Hendriks and Toerien, 1973; Theiler, 1989; Kaufman, In Postimplantation Mammalian Embryos: a Practical Approach (ed. A. J. Copp and D. L. Cockroft) pp. 81-91. New York: Oxford University Press, 1990).
The developmental function of tyrosine kinases during axonogenesis has been studied in Drosophila. A function in axonal pathfinding is evident for the Drosophila abl tyrosine kinase when abl mutations are combined with mutations in other genes including the neural cell adhesion molecule, fasciclin I (fas I, Elkins et al., Cell 60:565-575, 1990) or disabled (dab, Gertler et al., Cell 58:103-113, 1989). These studies have shown that the abl tyrosine kinase is specifically localized to the axonal compartment of the embryonic Central Nervous System (CNS) (Gertler et al., Cell 58:103-113, 1989). Moreover, genetic analysis has indicated that subcellular localization to axons is essential for abl function during development (Henkemeyer et al., Cell 63:949-960, 1990) and that mutations in second-site modifier genes including fas I and dab can reveal a role for abl in axonogenesis (Elkins et al., Cell 60:565-575, 1990; Gertler et al., Cell 58:103-113 1989). The requirement for tyrosine phosphorylation in axonal outgrowth and adhesion in Drosophila is strengthened by the identification in CNS axons of three transmembrane tyrosine phosphatases containing FN III motifs (Tian et al., Cell 67:675-685, 1991; Yang et al., Cell 67:661-673, 1991).